A typical planetary gearbox comprises a central pinion (sun), a central ring (orbit), and a group of offset pinions (planets) which ride on a carrier. It has advantages over an offset gearbox that include coaxial drive axes, high load capacity, and low pitch velocity (noise). These come at the expense of the production cost, friction, and reverse bending that result from the orbit ring. In addition, geometric constraints limit the minimum reduction ratio to 2:1 theoretically, and approximately 5:2 practically.
Variations on the planetary gearbox are plentiful and include the strain wave, cycloid, and nutating gearboxes. These configurations are more complex, less efficient and are typically used to satisfy special requirements such as anti-backlash or a high ratio.
The exemplary embodiments disclosed herein resemble an epicyclic gearbox with its orbit ring replaced by a second carrier which is cheaper to produce, creates less friction, and does not introduce reverse bending. A reduced pitch velocity generates less vibration and noise for improved high speed operation, and a minimum theoretical reduction ratio of 1:1 enables practical ratios of 5:4 or lower.